Chapter 10 Install and System Administration for FortiOS 5.0 : Using the web-based manager : Dashboard : System Information widget : Formatting USB
  
Formatting USB
The FortiGate unit enables you to back up the configuration of the device to a USB flash drive. The USB flash drive must be formatted as a FAT16 disk.
To formate the USB flash drive, either use the CLI command exe usb-disk format. or within Windows at a command prompt, enter the command
format <drive_letter>: /FS:FAT /V:<drive_label>
where <drive_letter> is the letter of the connected USB flash drive and <drive_label> is the name to give the USB drive.